The demand exists for coatings that are resistant to contamination. In particular, in the field of optics, these coatings not only need to resist contamination but also must be very thin with very low absorption such that the optical properties of the substrate, to which the coatings are applied, are not adversely affected. Typical optical coatings are prepared from various polymeric materials such as poly(fluoroalkyl)methacrylate and copolymers thereof. One such example is that of Ohmori et al. (U.S. Pat. No. 4,500,694) which is a copolymer consisting essentially of a fluorine-containing methacrylate monomer unit and a fluorine-containing acrylate monomer unit. This copolymer has marked improved thermal resistance, high transparency, low refractive index and high flexibility. These copolymers have molecular weights ranging from about 200,000 to about 4,000,000 and are typically used in injection molding processes. Although these copolymers may be used for coating compositions, the ability to apply them uniformly on a nanoscale level (ranging from 0.1 nanometers to about 200 nanometers) is unlikely.
Thünemann and Lochhaas reported a fluorinated complex that can be used as a coating material for smooth surfaces which does not affect the appearance of the surface (Andreas F. Thünemann and Kai Helmut Lochhaas, Surface and Solid-State Properties of a Fluorinated Polyelectrolyte-Surfactant Complex, 15 Langmuir, Apr. 7, 1999, at 4867.). This coating material is a highly ordered mesomorphous polyelectroltye-surfactant complex capable of repelling oil and water. The complex, itself, is insoluble in water. However, when the complex comes into contract with water, it displays a dramatic surface reconstruction, which is reversible when redried. This reconstruction results in an increase in surface energy, causing the surface to repel oil and water. Although these films demonstrated the desired ability to repel oil and water, the teaching in this article suggests that they must be applied at thicknesses between about 0.1 mm to about 1.0 mm, which is too thick for optical and other thin-film applications. Lin et al. found that cross-linking immobilizes the oriented perfluoroalkyl groups when forming a nonstick surface (Jun Lin, Jiayi Zhu, Douglas R. Swanson and Larry Milco, Cross-Linking and Physical Characteristics of a Water-Based Nonstick Hydrophobic Coating, 12 Langmuir, Dec. 1, 1996, at 6676.). Thus, cross-linking actually impedes water-induced structural rearrangement in the coating. However, these films are about 1.5 microns to about 2.1 microns thick, making them unsuitable for optical or thin-film applications. In each of these instances, the polymer is already formed prior to surface application.
Schmidt et al. prepared a class of water-based non-stick coatings using self-assembly and immobilization of reactive polymeric surfactants containing pendant perfluoroalkyl groups which become oriented to yield surfaces with very low energy (Donald L. Schmidt, Charles E. Coburn, Benjamin M. DeKoven, Gregg E. Potter, Gregory F. Meyers, and Daniel A. Fischer, Water-Based Non-Stick Hydrophobic Coatings, 368 Nature, Mar. 3, 1994, at 39.). Various reactive perfluoroalkyl polymeric surfactants were prepared by copolymerizing acrylate or methacrylate esters of fluoroalkyl alcohols with carboxylic acid functional vinyl monomers. The formulation, comprising a 10:10:80 weight ratio of solids, ethylene glycol, and water were cast and cured on microscope slides. The final films were cross-linked to prevent attack by polar solvents. Schmidt et al. disclose various examples of the surfactants in U.S. Pat. No. 5,006,624. The resulting films are much thicker than the thickness suitable for the present invention where uniform application of the coating within the nanoscale range (0.1 nanometers to about 200 nanometers) is desirable.
Miller (U.S. Pat. No. 6,270,946) discloses a process for producing nanoscale features on a substrate. The process involves the selective application of a first difunctional molecule to the surface of a substrate and allowing it to react with the substrate. A second difunctional molecule is applied and reacted with the unreacted functional groups from the first difunctional molecule to form a patterned layer on the surface of the substrate. The selective application was accomplished by using a nanoscale delivery device. This process results in the formation of nanoscale features in three directions (x, y and z) at selective locations on the substrate. The problem with this method is that gaps are left on the surface of the substrate, causing the substrate to remain unprotected. In addition, this process is not amenable to mass production because of the selectivity required in applying the first difunctional molecule.
An object of the present invention is to prepare a coated substrate using self-assembled techniques such that each bilayer has a thickness ranging from about 0.1 nanometers to about 20 nanometers.
Another object of the present invention is to prepare a uniformly coated substrate that is hydrophobic and/or oleophobic.
Another object of the present invention is to prepare a uniformly coated substrate wherein the coating has very low absorption (wavelengths ranging from about 300 nm to about 3000 nanometers).